The continuous galvanization methods for thin sheet are well known and at present enable high quality production with an apparatus which is technologically adapted to the operations in question.
Some of these techniques, whatever their intrinsic qualities or particular features, relate specifically to galvanization on both faces. This is carried out in practice by continuously passing steel strip through an immersion tank.
Other methods relate to the galvanization of a single face without leaving any traces of zinc on the other face. By way of example, the thin sheet designed in particular for the manufacture of vehicle bodywork is included in this category, as the non-galvanized surface is designed in particular to receive a paint coating. In this field, there is already proposed a method in which steel strip is brought into contact with a rotary masking cylinder having a horizontal axis which is partially immersed in a bath of liquid zinc. As a result of winding about a portion of the periphery of the masking cylinder, the strip is introduced into the bath under the following conditions: the contact between the strip and the cylinder is established before the introduction of the strip into the bath and is only discontinued after the strip has emerged from the bath. (see British Patent Specification No. 1,533,193).
Up to now, these two types of coating lines have been constituted by specific apparatus which may not be used, even partially, for both types of application.